Support release version numbers for start/end range

[ehuss]

Problem

Often I know a regression happened between two specific stable versions of Rust. However, it is not terribly easy to use cargo-bisect-rustc with a range of those two releases.

Proposed Solution

Support release tags in --start and --end (in addition to dates and hashes).

I think the commit can be found with git merge-base master $TAG, though I'm not certain that will always work.

[Kobzol]

I tried to add support for resolving tags and it can be done quite easily by using revparse_ext instead of revparse_single for parsing revisions for the local resolves (the GitHub backend can resolve tags directly). Then I realized that resolving tags is not the problem at all :D The problem is that release commits are not in master. I sent a PR: #147

[Kobzol]

Should be resolved by #147.

[ehuss]

Yep, thanks!